Date of Award


Document Type


Degree Name

Master of Science (MS)


Biological Sciences

Committee Chair/Advisor

Kara E. Powder

Committee Member

William Baldwin

Committee Member

Susan Chapman

Committee Member

Samantha Price


Explaining the underlying source that generates the phenotypic diversity ubiquitous in nature has been a major goal of biologists since the time of Darwin. In addition to the phenotypic variation that exists between different species, another potent source of morphological diversity is found within a species, in the form of phenotypic differences that are specific to males and females, termed sexual dimorphism. The largest craniofacial bone, the mandible, exhibits such extreme sexual dimorphism that it may be used to forensically determine sex. This dimorphism occurs in most of the craniofacial complex and contributes to sex-biased disease frequencies, such as a higher prevalence of temporomandibular joint (TMJ) disorders in females and higher rates of cleft lip and/or palate in males. The developmental timing of these male-female differences, and the molecular mechanisms that facilitate them, are unknown. Lake Malawi cichlids are an attractive model system for studying craniofacial development, due to their recent adaptive radiation which has produced several species with a range of diverse craniofacial phenotypes. To understand the evolutionary and developmental influences that drive species-specific craniofacial sexual dimorphism, we quantified the degree of sexual dimorphism in the lateral and ventral mandible, as well as the lateral and ventral overall head shape in five cichlid species, using geometric morphometrics. We observed sexual dimorphism in all but the ventral mandible, and these sex differences were species-specific. We also determined the serum and gonad concentration of estradiol and progesterone in two cichlid species for the first time and found that they were sexually dimorphic and occurred in species-specific patterns. Finally, we manipulated estradiol and hormone signaling in embryos at various developmental stages and quantified their phenotypic effects with geometric morphometrics. We discovered that estradiol affects the development of the craniofacial skeleton during the windows of bone and cartilage progenitor cell migration, while progesterone affects development at later stages, mainly during early chondrogenesis. Taken together, we were able to gain a further understanding of how sex hormone signaling in early craniofacial development translates into species-specific sexual dimorphism, and these results allow us insight into the potential evolutionary pressures driving these phenotypic differences between males and females.

Author ORCID Identifier




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